The present invention is related to an improved method of forming a solid electrolytic capacitor and a capacitor formed thereby. More specifically, the present invention is related to a capacitor with improved reliability due to minimization of the degradation that typically occurs in the cathode during reformation of the dielectric.
The process of forming a solid electrolytic capacitor is well documented. In general, the process includes the formation of a dielectric on an anode followed by the formation of a solid conductive material on the dielectric. Though manganese dioxide is a well known solid conductive material, the instant invention is specific to electrically conductive polymeric cathodes such as polyaniline, polypyrrole, polythiophene and their derivatives. Electrically conductive polymeric cathodes have proven to be very effective due to their low equivalent series resistance and non-burning, non-ignition, failure mode. The electrically conductive polymeric cathodes are not without limitations and mitigation of one limitation is provided herein.
Electrically conductive polymeric cathodes are formed by either an in-situ process, including chemical or electrochemical polymerization of monomers, or by coating with a pre-formed conductive polymer dispersion. In-situ polymerization is particularly preferred for the initial polymer layers due to the increased penetration of conductive polymer into the interstitial regions of the porous pellet. Unfortunately, the in-situ polymerization degrades the dielectric. While not limited to any theory, it is hypothesized that the dielectric is degraded during in-situ polymerization of conductive polymer due to the interaction between high-energy radicals, from the polymerization process, and the dielectric surface. U.S. Pat. No. 7,563,290 describes some consequences of dielectric degradation by in-situ polymerization of conductive polymer, including reduced voltage withstanding capability, as indicated by a decrease in breakdown voltage.
Due to the aforementioned dielectric degradation it has become common in the art to subject the dielectric to a reformation step after polymer layer formation wherein the capacitor is immersed in an electrolyte solution with application of a DC voltage below that used in dielectric formation. This has long been the standard procedure for manganese dioxide based capacitors, as described in U.S. Pat. No. 2,936,514, and has been adapted for use in conductive polymer based capacitors. In dielectric reformation a diluted aqueous acidic solution; such as phosphoric acid, sulfuric acid or nitric acid; can be used for dielectric reformation with an applied DC voltage that is 60-85% of the dielectric formation voltage as described, for example, in U.S. Pat. No. 6,136,176. The acids are preferably the organic acids whose anions are used as dopants in the conductive polymer, such as toluenesulfonic acid, dodecylbenzensulfonic acid, and camphorsulfonic acid.
Unfortunately, the dielectric reformation process, done to counteract the dielectric degradation resulting from the cathode formation, degrades the polymeric cathode thereby creating localized non-conductive regions in the conductive polymer layer which reduces the current capacity of the conductive polymer layer. While not limited to theory, it is hypothesized that during the dielectric reformation process oxidation of conductive polymer occurs and the heat generated by the oxidation further degrades additional polymer leading to, under some conditions, catastrophic breakdown or localized damage to the dielectric. The skilled artisan was therefore bound by, on one extreme, inferior dielectric resulting from the formation of the conductive polymer, and on the other extreme, by poor performance due to polymer oxidation resulting from the reformation of the dielectric. The instant invention provides a solid electrolytic capacitor with improved reliability by locally controlling the electric current and heat generation during reformation thereby allowing for further dielectric reformation without degradation of the conductive polymer.